M2 Transport across the Membrane

Question 1

What are the 5 main things membranes consist of?

Answer 1

• Phospholipids
• Proteins
• Cholesterol
• Glycolipids
• Glycoproteins

Question 2

Describe the function of phospholipids in cell membranes.

Answer 2

• Form a bilayer.
• Allow transport of lipid soluble substances and very small molecules through the membrane.
• Do not allow transport of water soluble (polar) substances and ions through the membrane.
• Give membrane flexibility.

Question 3

What are the 2 different types of proteins in cell membranes.

Answer 3

Extrinsic (on the surface)
Intrinsic (span the bilayer from one side to another)
- carrier proteins and channel proteins

Question 4

What are the roles of proteins in cell membranes?

Answer 4

• Provide structural support
• Form cell surface receptors for cell recognition
• Adhesion of cells
• Receptor sites e.g. for hormones

Question 5

What is the role of cholesterol in cell membranes?

Answer 5

• reduces lateral movement, providing strength and stability.
• reduces fluidity at high temperatures.
• prevents leakage of water and dissolved ions from the cell.

Question 6

What are the roles of glycolipids and glycoproteins?

Answer 6

• act as recognition sites
• maintain stability of membrane
• help cells attach to form tissues
• glycoproteins specifically form receptors for hormones and neurotransmitters

Question 7

What is diffusion? DEFINITION

Answer 7

The net movement of a substance down a concentration gradient.

Question 8

Describe the two proteins involved in facilitated diffusion.

Answer 8

Protein channels:

• Hydrophilic channel through the phospholipid bilayer.
• Permanently open

Carrier proteins:
- Molecule binds to proteins, protein changes shape and molecule is released inside of the cell.

Question 9

What are the factors that affect the rate of Diffusion across cell membranes?

Answer 9

• Surface Area
• Differences in Conc Gradient
• Number of Channel/carrier proteins

Question 10

What is osmosis? DEFINITION

Answer 10

The net movement of water molecules from an area of high water potential to an area of lower water potential through a partially permeable membrane down a water potential gradient.

Question 11

What is water potential?

Answer 11

Pressure caused by water molecules moving around and colliding with the cell membrane.

Question 12

What has the highest water potential? How do you measure it?

Answer 12

Pure water = 0kPA

the more negative the number the lower the water potential.

Question 13

What is solute potential?

Answer 13

The amount the water potential of a solution is lowered by when solutes restrict the movement of water molecules (due to hydration shells).

Question 14

What is pressure potential?

Answer 14

The positive contribution made by pressure to water potential (e.g. plant cell walls), increasing water potential.

Question 15

What is the equation for water potential in plant cells?

Answer 15

Water Potential = Solute Potential + Pressure Potential

Question 16

What kind of water potential do hypertonic, isotonic and hypotonic solutions have?

Answer 16

Hypertonic: Higher water potential in the cell than in the solution (cell is plasmolysed)

Isotonic: The same water potential

Hypotonic: Lower water potential in cell than in solution (cell is turgid/bursts)

Question 17

What is incipient plasmolysis?

Answer 17

When the membrane pulls away from the cell wall so it is no longer pressing on it (the pressure potential of the cell drops to 0).

Question 18

What happens in a plant cell when it is turgid?

Answer 18

The membrane is pressing against the cell wall.

Question 19

How do you know under a microscope if a solution is isotonic?

Answer 19

50% of the cells are plasmolysed.

Question 20

What is the definition of active transport?

Answer 20

The movement of molecules/ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins.

Question 21

Describe the process of active transport.

Answer 21

1) Molecule/ion binds to a receptor site on one side of the carrier protein.
2) On the inside of the cell, ATP binds to the carrier protein, which acts as an enzyme (ATP -> ADP + Pi)
3) This causes the carrier protein to change shape. The Pi is still bound to it at this stage.
4) The molecule/ion is released to the other side of the membrane.
5) The phosphate group is released and the carrier protein returns to its original shape.

Question 22

Carrier proteins are highly ____, so __________ binding occurs.

Answer 22

Specific

Complementary

Question 23

Describe the process of co-transport in the ileum.

Answer 23

1) Na+ are actively transported out of the cell into the bloodstream - low Na+ conc in the cell.
2) Na+ diffuse into the cell, down the conc gradient through a co-transport protein taking glucose with it - high glucose conc in the cell.
3) Glucose leaves cell by facilitated diffusion and enters the bloodstream.

Question 24

Other than glucose, what is also co-transported with Na+?

Answer 24

Amino Acids

Question 25

How do you determine the solute potential of a plant cell?

Answer 25

Place plant tissues in different concentrations of salt or sugars. Observe using a microscope the concentration in which 50% of cells are plasmolysed (isotonic).